Computers and related devices are commonly networked together using known networking schemes to exchange packets or frames of data (often referred to as data packets, data frames, or datagrams). Computers connected to a network are often referred to as hosts. On the Internet, the term host generally means any computer that has full two-way access to other computers on the Internet. Computers tied to a network employ some kind of a connection device, such as a network interface card (NIC) to communicate and exchange data with the network.
A variety of different network types exist. For example, a wide area network (WAN) is a geographically dispersed telecommunications network. A WAN may be privately owned or rented, but the term usually is construed to include public (shared user) networks. A WAN is distinguished as a broader telecommunication structure from that of a metropolitan area network (MAN), which is larger than a local area network (LAN). A LAN is a network of interconnected workstations sharing the resources of a single processor or server within a relatively small geographic area (e.g., an office building). Some common LAN technologies include: Ethernet, token ring, Attached Resource Computer Network (ARCNET), and Fiber Distributed Data Interface (FDDI). A LAN may serve only a few users, or by using FDDI, may serve several thousand over a much wider area. The server will usually have applications and data storage that are shared in common by multiple workstation users.
A LAN server can also be used as a Web server on the Internet if safeguards are taken to secure internal applications and data from outside access. On the Internet, certain protocols are used, including Transmission Control Protocol (TCP) and Internet Protocol (IP). TCP uses a set of rules for exchanging messages with other Internet connections at the information packet level. IP uses a set of rules to send and receive messages at the Internet address level. For IP Version 4, an IP address consists of a 32-bit number and is used to identify each sender and receiver of information that is sent in packets across the Internet. In contrast, IP Version 6 uses 128-bit addresses, and future versions will likely use even more bits to facilitate higher data traffic flows. TCP/IP assigns a unique number (or “IP number”) to every workstation. This IP number is a four-byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. An IP address has two parts: the identifier of a particular network on the Internet (first two bytes) and an identifier of the particular device (which might be a server or workstation and is the second two bytes) within that network. The range of IP addresses is divided into “classes” based upon the high order bits within the 32-bit IP address. Class A addresses are for large networks with many devices, class B addresses are for medium-sized networks, class C addresses are for small networks (e.g., fewer than 256 devices), class D addresses are multicast addresses, and class E addresses are reserved.
The Open Systems Interconnection (OSI) standard is a system for transmission of messages between two points in a telecommunication network. The communication is divided into layers, with each layer adding certain additional functionality. For any given message, there will be a flow of data down through each layer at a sending point. At the receiving point, the data will flow up through the layers. The programming and hardware that furnishes these layers of functionality usually results from a combination of the computer operating system, applications, and TCP/IP or other network protocols. OSI divides a communication into seven layers. The layers may further be divided into two groups, where the upper four layers are used when a message passes from or to a user, and the lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host computer are not passed up to the upper layers but are forwarded to another host.
The seven OSI layers include: Layer 1, the physical layer, which conveys the bit stream through the network at the electrical and mechanical level; Layer 2, the data link layer, which provides error control and synchronization for the physical level; Layer 3, the network layer, which handles the routing and forwarding of data; Layer 4, the transport layer, which manages the end-to-end control and error-checking of data transfer; Layer 5, the session layer, which sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end; Layer 6, the presentation layer, which is usually part of the operating system and converts incoming and outgoing data from one presentation format to another (e.g., syntax layer); and Layer 7, the application layer, which is where communication partners are identified, quality of service is identified, user authentication and privacy are considered, and constraints on data syntax are identified. Referring again to Layer 2, this protocol layer handles the moving of data in and out across the physical link in a network. Layer 2 contains two sublayers: the Media Access Control (MAC) and the Logical Link Control (LLC). On a LAN, for example, messages are sent between machines by supplying a six-byte unique identifier, which is referred to as the MAC address.
Referring again to IP addresses belonging to Class D, a multicast address is used to send data to a large number of hosts on the Internet, rather than to a specific individual user. Not all hosts will actually use or process such data. Analogized to a radio or TV broadcast, the receiving party must have “tuned” their computer device to receive such data. For multicast addresses (in a 32-bit system), the lowest 4 bits are “1110” followed by the 28-bit multicast address. Hence, every IP datagram whose destination starts with “1110” is an IP multicast datagram. The machine or physical address (e.g., MAC address) used within the local area network of an organization may be different than the IP address on the Internet. A typical example is the 48-bit Ethernet address (with Ethernet being a widely installed LAN technology). TCP/IP includes a facility called the Address Resolution Protocol (ARP) that lets the network administrator create a table that maps the 32-bit (4 byte) IP addresses to the 48-bit (6 byte) MAC addresses. On an Ethernet LAN, the MAC address is typically the same as the Ethernet address.